Relative carbon content meter for smoke filters

ABSTRACT

An apparatus to measure the relative quantity of carbon present in a smoke filter comprising a parallel plate capacitor between whose parallel plates are disposed rods of filter material. The parallel plate capacitor is attached to a capacitance measuring device which is calibrated to measure a capacitive change of the system comprising said parallel plates and said rods and to indicate when the carbon content of said rods deviates from a predetermined standard. An embodiment of the invention is disclosed which comprises an improved device for measuring the capacitance of said capacitive system by utilizing a linear approximation in the dependency of collector current with small variations in capacitance added to the base circuit in a tuned collector, tuned base transistor oscillator wherein said dependency is measured in a &#39;&#39;&#39;&#39;Wheatstone&#39;&#39;&#39;&#39; bridge circuit.

United States Patent Pullman et al.

[ 51 Oct. 3, 1972 [54] RELATIVE CARBON CONTENT METER FOR SMOKE FILTERS[73] Assignee: Liggett & Myers Incorporated, New

York, NY.

22 Filed: Feb. 4, 1970 211 Appl. No.: 8,603

[52] US. Cl. ..324/61 R, 331/65 [51] Int. Cl. ..G01r 27/26 [58] Field ofSearch ..324/61, 61 P, 61 B, 61 T1;

' [56] References Cited UNITED STATES PATENTS I 3,090,004 5/1963 Breenet a1. ..324/61 1,932,337 10/1933 Dowling ..324/6l 1,984,166 12/1934Walter ..324/61 P 3,482,162 12/1969 Wochnowski ..324/61 3,182,255 5/1965Hopkins et al ..324/61 2,576,489 11/1951 Stovall ..324/61 Tl Senm..324/6l Tl Erwin ..324/61 B Primary Examiner-Alfred E. SmithAttorney-Kenyon & Kenyon Reilly Carr & Chapin [S 7] ABSTRACT Anapparatus to measure the relative quantity of carbon present in a smokefilter comprising a parallel plate capacitor between whose parallelplates are disposed rods of filter material. The parallel platecapacitor is attached to a capacitance measuring device which iscalibrated to measure a capacitive change of the system comprising saidparallel plates and said rods and to indicate when the carbon content ofsaid rods deviates from a predetermined standard. An embodiment of theinvention is disclosed which comprises an improved device for measuringthe capacitance of said capacitive system by utilizing alinearapproximation in the dependency of collector current with smallvariations in capacitance added to the base circuit in a tunedcollector, tuned base transistor oscillator wherein said dependency ismeasured in a Wheatstone bridge circuit.

27 Claims, 10 Drawing Figures PATENTED 0013 I972 3.696. 299

SHEET 1 OF 5 [NVENTORS JAMES PJLWA/Y H 7 DEA 67S PATENTEDncra m23,696,299

SHEET u 0F 5 I'LbA i SA.

/ NVENTORS James O. Pam 14 5y Mum/*1 P. 14 5/0001 RELATIVE CARBONCONTENT METER FOR SMOKE FILTERS BACKGROUND OF THE INVENTION 1. Field ofthe Invention This invention relates to an apparatus for measuring thecarbon content of smoke filters and more specifically relates to anapparatus which measures said carbon content by electronic means andthus non-destructively.

2. Description of the Prior Art The primary components of conventionalcigarette filters typically are plugs composed of paper or celluloseacetate filter tow, plasticizer and wrapping paper. Of these, the tow issusceptible to variations which may seriously effect the filters. Tomaintain filter quality, it is customary to control the weight of thefilter rods which are produced. This will effectively restrictvariations in tow weight, and, hence, in pressure drop through thefilter, which is a property of the finished cigarette which is of basicimportance to the consumer.

Certain smoke filters comprise carbon as well as the above mentionedcomponents. Generally, these filters comprise a carbon-bearing regionbounded on at least one end by a non-carbon bearing region. For example,one commercial-filter of this kind has a carbon bearing region whichconsists'of a chamber partially filledwith activated carbon granules,bounded on each end by a piece of conventional cellulose tow. Themanufacture of these filters is more difficult to control because thereare now two components, i.e., tow and the carbon, either of which mayvary appreciably in weight. In the past, variations in tow and carbonwere regulated by a system of control by over-all weight. Occasionally,tow tests were made in which the carbon feed was shut off, and the totalweight of the non-carbon components was set to within a prescribedrange. A specific weight of carbon was then added, and the over-allweight of the tow plus the carbon was then maintained within prescribedlimits. Although practicable, this mode of control led to difficulties.The .tow tests produced unusable waste and so the number of such testshad to be minimized. Yet, with any reasonable interval between towtests, changes in the over-all weight of the tow and the carbon werebound to occur. When such changes occurred, it had to be decided whetherit was the carbon or whether it was the tow which led to such variationand which one had to be corrected. There was often little basis forchoice and, in practice, a variation in one of the components was oftenmiscompensated for by an adjustment to the other. The result was thatthe over-all filter weight was accurately maintained, but that theindividual tow and carbon weights varied widely.

It is of course desirable that the over-all filter weight be maintained,but only within rather broad limits. If cigarettes made with the smokefilters comprising carbon are to appear uniform to the consumer, it ismore important to individually control the filters tow weight, whichdetermines the pressure drop across the filter or what is commonly knownas the draw, and to im dividually control the carbon content whicheffects the gas phase removal through the filter and thus effects whatis commonly called the smoothness.

Therefore, a new control system for smoke filters comprising carbonbased directly on these consumer related parameters was needed.Techniques have already beendeveloped for measuring the filter pressuredrop or the tow weight referred to above. The present invention isdirected toward an apparatus which indicates the carbon content of smokefilters in a manner which is non-destructible to said filters.

SUMMARY OF THE INVENTION This invention measures the relative quantityof carbon present in a smoke filter. This is accomplished by utilizing aparallel plate capacitor and introducing between theplates of saidcapacitor rods of smoke filters comprising carbon. The introduction ofthe carbon contained in the rods of smoke filters between the plates ofsaid capacitor will change the capacitance which can be measured betweensaid plates, and the magnitude of the change will depend upon the amountof carbon contained by said filters. Such a system is calibrated byintroducing filters containing varying amounts of carbon between theplates of the capacitor and recording the amount of capacitance measuredacross the parallel plates for each of said filters. Said filters arethen disassembled and the carbon content determined directly. A table ofcarbon content versus capacitance change is then established. Thus acapacitance range corresponding to therange of carbon content of smokefilters which is acceptable for commercial use is specified. Filterswhich produce a capacitance change outside said specified range will berejected as having an unacceptable carbon content.

An embodiment of this invention comprises a unique device for measuringthe capacitance of a parallel plate capacitor with the smoke filter rodscontaining carbon disposed therebetween. This device utilizes a linearapproximation in the dependency of collector current on small variationsin capacitance added to the base circuit in a tuned collector, tunedbase transistor oscillator wherein said dependency is measured in aWheatstone bridge circuit.

It is thus an object of the present invention to provide a device whichconsistently and accurately measures the relative carbon content ofsmoke filters.

It is a further object of this invention that said measurement of therelative carbon content of smoke filters shall be non-destructible tosaid filters.

It is a further object of this invention to utilize a device which isrelatively simple and economical in operation to measure the capacitanceof a system which relates the capacitance of a system with the relativequantity of carbon in the smoke filters to be tested.

This invention can be applied to many different types of smoke filterswhich comprise carbon in a carbon bearing region bounded on at least oneend by a noncarbon bearing region. The greatest difficulties are encentfill, of the chambers, referring to the relative volume of the chamberswhich is occupied-by carbon granules, and to refer to the embodiment-ofthe invention used with said free carbon-type filters as a percent fillmeter. Carbon content may equally well be expressed in terms of weight,as would be appropriate with othertypes of smoke filters comprisingcarbon.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description of anembodiment of the invenf tion to test the carbon content infilter rodsof the free carbon-type, taken in connection with the accompany- FIG. 1is'a view of the invention illustrating a housing for a parallel platecapacitor with a vibrator attached thereto and mounted on flat leafsprings as well as a housingfor capacitance measuring and indicatingcontrols.

FIG. 2 is a partial view of the parallel plate capacitor housing withthe upper ridged plate in the open position and showingrods ofcarbon-type filter material disposed therein.

FIG. 3 is an underside view of the parallel plate capacitor housingshowing the bottom side of the lower plate of the parallel platecapacitor.

FIG. 4 is a sectional view of the parallel plate capacitor housingcomprised in the present invention.

FIG. 5 is a sectional view of FIG. 4 taken along the line 5-5 in FIG. 4in the direction of the arrows.

FIG. 5a is a sectional view of an alternate embodiment of the invention.

FIG. 6 is a schematic view of a tuned base, tuned collector transistoroscillator circuit used in an embodiment of the invention.

FIG. 7 is a schematic view of a Wheatstone bridge circuit with thecircuit of FIG. 6 incorporated therein.

FIG. 8 is a graph illustrating the operation of a portion of anembodiment of the invention.

FIG. 9 is a schematic view of an embodiment of the circuit illustratedin FIG. 7.

DETAILED DESCRIPTION tion material, houses a parallel plate capacitorhousing 1 designated generally by 104 and a capacitance measur--:

ing and indicating control housing "106.'-The parallel plate capacitorhousing 104 comprises side walls 108 and 110 and end walls 112 and 114.In the preferred form of the invention shown in the drawings, side walls108 and 110 and end walls 112 and 114 are comprised of a metallicmaterial and are fastened together by welding, internal to the housing104. A base portion 116 of housing 104 is also comprised of a metallicmaterial which is fastened to the side walls 108 and 110 and the endwalls 112 and114 by means of welding internal to housing 104. Base 116has lip portions 118 and-120 which extend beyond the side walls 108 and110. A face plate 122 comprises the top portion of housing 104. Faceplate 122 is also comprised of a metallic material which is attached toside walls 108 and 1 10 and end walls 112 and 114 by means of metalscrews 124.

An upper capacitor plate 126 of the parallel plate capacitor housed inhousing 104 is rotatably mounted on the face plate 122 of housing 104.Bearing housings 128 and 130 are mounted on face plate 122 by means ofmetal screws 131and 132, respectively. Precision shaft and instrumentbearings 133 and 134 are respectively housed in bearing housings 128 and130. Block 136 is rotatably mounted in bearings 133 and 134 by means ofa shaft 138. Furthermore, block 136 is attached to capacitor plate 126by means of screws 140 (FIG. 4). A toggle clamp type of latch 142 ismounted on capacitor plate 126. The latch 142 has a handle 144 whichactuates the latch into and out of engagement with a stationary lug 146which'is mounted by means of a metal screw 147 and a pin 148 on faceplate 122 of housing 104 (FIG. 4).

Thus, it may be seen that the upper capacitor plate 126 of theparallelplate capacitor housed in housing 104 is rotatably mounted inthe face plate 122 of housing 104 so that when said capacitor plate 126is in the closed position (as shown in FIG. 1 and FIG. 4) the latch 142engages the lug 146 and the upper surface of plate 126 is contiguouswith the upper surface of plate 122 of housing 104.

Housing 104 is mounted to a fixed portion 150 of test stand 102 by fourspring supports designated generally at 152a, 152b, 152a, and 152d (FIG;1, 152d not shown), each distending from a corner of base 116 of housing104. Each spring support l52a-d comprises a spacer block 154a-d of woodor any other suitable material which is attached at one end to thecorners of base 116 of housing 104 by-means of bolts 156a-d. Leafsprings 158a-d. are attached at one end to the other end of spacerblocks 154a-d by means of bolts 156a.d and nuts 160a-d. The other endsof leaf springs l58a-d are attached at their other ends to one end ofspacer blocks 162a-dby means of bolts l64a-d. Leaf springs 166a-d areattached at one end to the other ends of spacers 162a-d by means ofbolts 164a-d and nuts 168ad. The other ends of leaf spring 166a-d areattached to one end of spacers 169a-d by means of bolts 170a-d. Spacers169a-d are connected at their other end to a base plate 171 by bolts170a-d and by recessed nuts on the underside of plate 171 (not shown).Plate 171 is fastened to the stationary portion 150 of test stand 102 bymeans, of fasteners 172.

For purposes hereinbelow described, a vibrator 174 is attached to theunderside of base plate 116 of housing 104. The vibrator used ina'preferred embodiment of thepresent invention is a commercial vibratormanufactured by the Syntron Company and known as Syntron vibrator modelV4AC. It is to be noted that any form of spring supports 152a-d could beused with the invention, for example, flexible rubber spring supportscan also be used.

Housing 106 houses capacitance measuring and indicating circuits whichare described hereinbelow in connection with FIGS. 6-9. These circuitsmeasure and indicate the capacitance of the parallel plate capacitorwhich is mounted in housing 104. The housing 106 has provisions for apower-on switch 176 which controls the AC power. A switch 178 is themode switch which can be pressed to place the instrument in thecalibrate mode" for the purposes of verifying the calibration of theinstrument. The mode switch 178 can be pressed again to place theinstrument in the operate mode which is the mode used for the actualtesting or measurements of the carbon content of the free carbon-typefiltersto be tested. Switch 180 is the cycle switch which starts theautomatic operating cycle described hereinbelow. Meter 182 indicates thepercent deviation of the carbon content of the carbontype filtermaterial to be tested froma prescribed zero percent fill condition.Needle 184 indicates the percentage of deviation from this standard.Lever switch 186 places the operation of the meter 182 to the right orto the left of the zero percent deviation condition. Knob 188 is thezero control knob and is used'to place the needle 184 at the zeropercent deviationposition on the meter 182 when the instrument is in thecalibrate mode.

FIG. 2 is a partial view of the parallel plate capacitor housing 104with the upper ridged plate 126 in the open position. FIG. 3 shows theunderside of covering plate 122 of housing 104. Referring to FIG. 3, thebottom capacitor plate 190 of the parallel plate capacitor housed inhousing 104 forms the base of a compartment which has side walls 192 and194 comprised of an insulating material which are attached to saidcapacitor plate 190 by means of fasteners 196, and end wall spacers 198and 200 which are comprised of an insulating material and are attachedat one end to face plate 122 of housing 104 by means of bolts 202 and tothe ends of capacitor plate 190 by means of bolts 202, insulatingwashers 204 and nuts 206. (See also FIGS. 4 and 5).

Referring to FIG. 2, thirty rods 208 of free carbontype filters to betested are shown disposed on top of capacitor plate 190 in thecompartment described hereinabove in relation to FIG. 3. A portion ofinsulating spacers 198 and 200 are just higher than the rods of freecarbon-type material 208. Each rod 208 of free carbon-type filter to betested comprises alternating charcoal chambers 210 and cellulose filtersegments 212. Each rod will ultimately yield six free carbon-typefilters, with each filter comprising a single carbon bearing regionwhich for the present description is charcoal chamber 210 bounded ateach end by a non-carbon bearing region 212. The upper capacitor plate126 comprises ridges 214 attached to a surface of capacitor plate 126 bymeans of fasteners 2.16. Ridges 214 are so positioned on capacitor plate126, with respect to each other, that when said capacitor plate 126 islocked into the closed position by latch 142 and lug 146, the ridges 214are supported at their ends by insulating spacers 198 and 200 and areadjacent the charcoal chambers 210 of the free carbon-type filter rods208 when said free carbon-type filter rods are disposed on the lowercapacitor plate 190 (See FIGS. 4 and 5). Although the ridges 214 are notnecessary to the invention they are preferred for the presentlydescribed embodiment for reasons hereinbelow explained.

FIGS. 4 and 5 show the upper capacitor plate 126 lockedinto the closedposition by latch 142 and lug 146. The rods 2080f free carbon-typefilters to be tested are shown sandwiched on top of the lower capacitorplate .190 between a portion of the insulating spacers 198 and 200. Theridges 214 of the upper capacitor plate 126 are supported at their endsby a portion of the insulating spacers198 and 200 and are adjacent thecarbon chambers 210 of the rods 208. It may be noted that when the uppercapacitor plate .126 is locked into the closed position by latch 142 andlug 146 it forms .an integral closure with the housing 104, thus.completely enclosing the lower capacitor plate 190. This shielding ofthe lower capacitor plate protects it from the proximity effects ofnearby objects and personnel and furthermore shields any RF radiationfrom this hot plate. Power is provided to the lower capacitor plate 190via electrical plug 218. The electrical plug 218 is shown in FIG. 5 as aradio frequency coaxial connector which is mounted on base plate 1200fhousing 104 by screws 220 and is attached electrically tothe lowercapacitor plate 190 at the point FIG. 5a shows an alternate form of theinvention which utilizes, in addition to the upper capacitor plate 126.,an upper capacitor plate 126a. The altemate upper capacitor plate 126ahas ridges 214a which adjoin thenon-carbon regions 212 of the freecarbon-type filter rods 208 when said alternate plate 126:: is in theclosed position as shown in FIG. 5a. The operation of this alternateform of the invention is described hereinbelow, but it is to be notedthat the alternate upper capacitor plate 126 (with ridges adjacent thecarbon chambers) and the upper capacitor plate 126a (with ridgesadjacent the non-carbon regions) can be hinged at opposite ends of theparallel plate capacitor housing 104 to be clamped over the filter rods208 one at a time. An alternate construction to accomplish the use ofalternate upper capacitor plates 126 and 126a is to use a single ridgedupper capacitor plate which could be offset along the filters by a fixedamount necessary to shift the ridges of said upper capacitor plate froma position adjacent the carbon chambers 210 of the rods 208 to aposition adjacent the non-carbon regions 212 of the rods208.Furthermore, a more advanced version of the invention using thealternate upper capacitor plates might combine both arrays of ridges ina single plate and the conversion of the signals from the two arrays toa single, compensated reading could be done electronically.

When the free carbon-type filter rods 208 are loaded onto the lowercapacitor plate 190 between the insulated spacers 198 and 200 and theupper capacitor plate 126 is locked into position by latch 142 and lug146, the carbon within the carbon chambers 210 is generally in a stateof disarray. Often, for example, the rods 208 will roll just prior toreaching their final positions, leaving the carbon surface of the carbonwithin carbon chamber 210 at an angle to the surface of the lowercapacitor plate 190, adjacent said rods, or to the surface of the ridges214, adjacent said rods. This angle will vary fromtime to time andreadings obtained from successive measurements of a given sample of freecarbon-type filter rods 208 will therefore vary accordingly. Sinceaccuracyand consistency are objects of this in vention it is necessarythat the carbon array not vary appreciably from measurement tomeasurement. This is accomplished by attaching a vibrator 174 to thehousing 104 and providing flexible mounts 152a-d to support theinsulating frame 104 in the test station 102 (See FIGS. 1 and 4). Thevibration of the-frame 104 prior to each measurement assists inmaintaining a constant carbon array from measurement to measurement.

OPERATION OF THE INVENTION late the expectedcapacitance change acrossthe plates of the capacitor as the object is introduced between theplates. Nevertheless, if several such objects of similar shape butvariable size are successively introduced between the plates of thecapacitor, the change in capacitance across the plates of the capacitoras each object is introduced and then withdrawn will relate to theobject size in each case. The carbon in carbon chamber 210 of freecarbon-type filter rods 208 is effectively a small conductive object. Ifthe carbon chamber 210 has a substantially constant length and crosssection, changes in the carbon percent fill will be reflected in changesof carbon height. Thus if we place the free carbon-type filter rods 208between the plates 126 and 190 of the parallel plate capacitor, thecapacitancechange measured across the plates 126 and 190 by thecapacitance indicating means described hereinbelow will be proportionalto the carbon content of the batch of free carbon-type filters beingmeasured.

There are three possible orientations of the free carbon-type filterrods 208 and the capacitor plates 126 and 190. The capacitor plates 126and 190 could be placed vertically with respect to a horizon line andthe rods 208 parallel to said line. With this arrangement of the plates126 and 190 and the rods 208, the maximum sensitivity occurs when thecarbon chambers 210 are about half full. Another orientation is when theplates 126 and 190 and the rods 208 are all placed vertically withrespect to a horizon line. With this arrangement of the plates 126 and190 and the rods 208, the response is linear at all carbon levels, butthe response is only moderate. For the embodiment of the inventionillustrated in the drawing the third orientation is used where theplates 126 and 190 and the rods 208 are all parallel to the horizonline. This orientation give's rapidly increasing sensitivity for filterswhich are substantially more than half full. That orientation is chosenwhich is best suited for the free carbon-type filters 208 to bemeasured.

In operation, the power-on switch 176 is put into the on position. Themode switch 178 is depressed to put the instrument in the calibratemode. The meter 182 should read zero (center scale) and if it does notthe meter should be adjusted by use of knob 188'to place the needle atthe zero position. Furthermore, while the instrument is in the calibratemode the lever switch 186 pressed to the right or to the left shouldresult in a meter reading whichis between pairs of fiducial marks to theright or to the left, respectively, of the ends of the meter readings ofmeter 182. Next, the mode switch 178 should be depressed to place theinstrument in the operate mode. A sample batch of free carbon-typefilter rods 208 is then loaded onto the parallel plate capacitor 190 asshown in FIG. 2. The upper capacitor plate 126 is then locked in placeby latch 142 and lug 146 as shown in FIG. 4. The cycle switch is thendepressed to start the automatic operating cycle which is controlled bya cam timer (not shown). The vibrator 174 will 'the'n'vibrate thehousing 104 for a short period of time, e.g., 5 seconds, settling thecarbon in the carbon chambers 210 of the rods 208. The vibrator 174 isthen automatically turned off and the indicating meter 182 is connectedfor a short period of time, e.g., 5 seconds, during which time a readingis observed. At the end of this 10 second cycle, the meter is againdisconnected. The latch 142 is then disengaged from lug 146 and theupper capacitor plate 126 is placed in the open position.

As an illustration of its operation, this form of the invention wasadjusted to provide 'a zero deviation (center scale) reading for filterswith chambers filled with carbon granules to the extent of 63 percent.Groups of filter rods, each group having a different carbon content,were then prepared. A reading of the above-described instrument was thenobtained for each group. Then the filters were cut open and the truecarbon content determined directly. Typical results were as follows:

Average relative carbon content of filters, percent filled Meterreading, percent fill deviation from 63% full When the above data areplotted, all points are within one percent fill of an average line.

It is to be noted that the'above description of the operation of thisinvention describes a specific use of the invention and that the scopeof the invention is not so limited. Furthermore, in the form of theinvention shown in FIGS. 2, 4 and 5, the upper capacitor plate 126comprises ridges 214. As mentioned above, although the invention maycomprise an upper capacitor plate 126 which has no ridges, the preferredform of the invention comprises ridges 214. The ridges 214 are used onthe upper capacitor plate 126 to aid in eliminating the response of theparallel plate capacitor to moisture variations in the non-carbonportions 212 of the free carbon-type filter rods 208. Since the ridges214 of the upper capacitor plate 126 adjoin the carbon chambers 210, asshown in FIG. 5, the efiect of the non-carbon regions 212 is made lessby widening the spacing between the upper capacitor plate 126 and thelower capacitor plate 190 in those regions. A further reduction may beaccomplished with an alternate construction of the lower capacitor platewhich has ridges adjoining the carbon chambers 210. Alternately, theinvention can comprise in addition to the upper capacitor plate 126which has ridges which adjoin the carbon chambers 210 when said plate isin the closed position, a second upper capacitor plate 126a which hasridges which adjoin the non-carbon regions 212 when said second plate isin the closed position as shown in FIG. 5a. The plate 126 is verysensitive to the carbon in the free carbon-type filter rods 208, butalso responds somewhat to the non-carbon materials in said rods. Theplate 126a, on the other hand, has a minimum response to the carbon inthe carbon chambers 210, and has a maximum response to the non-carbonregions 212. Thus, the capacitance across the parallel plate capacitoris measured for a given batch of free carbon-type filter rods 208 whenthe upper capacitor plate 126 is closed and again when the uppercapacitor plate 126a is closed. These two readings of capacitance,called R and R respectively, are thencombined to form a reading R=R KRwhich is independent of moisture. R is the reading of capacitance withthe upper capacitor plate 126 closed. R is the reading of capacitancewith the upper capacitor plate 126a closed. K is a constant which isdetermined empirically to cause the combination R KR to be moistureindependent. This alternate use of two upper capacitor plates has asmall loss in carbon sensitivity over the use of only the single upperplate 126, but the accuracy of the response to carbon is measurablyincreased.

Although the above description of the preferred form of the inventionwas in connection with the measurement of the carbon content of rods offree carbontype filters comprising alternating carbon chambers andnon-carbon regions, the present invention could also be used todetermine the carbon content of filters of the carbon-impregnatedacetate type in which the carbon bearing regions are carbon granulesdispersed in a rod of cellulose acetate filter material. For thismeasurement the vibrator 174 is not necessary and the upper capacitorplate 126 might be flat and not have the ridges 214. I

Further the orientation of the capacitor plates and filter rods wouldhave no effect on the response of the instrument to carbon and would bea matter of convenience only. When the parallel plate capacitor deviceof the invention as shown in FIGS. 1-5 was used with a modified form ofthe meter 182 to measure the carbon content of carbon-impregnatedacetate type filters in which no plasticizer was used and in which thecarbon was 14 X 40 mesh, typical results were as follows:

Sample Carbon Content Carbon Meter (gm/ I rods) Reading When the abovedata are plotted, all points lie within 1.0 gm/ 100 rods of an average,almost linear curve.

As mentioned above, the housing 106 houses a capacitance indicator meanswhich measures the capacitance of the parallel plate capacitorcomprising capacitor plates 126 and 190. This capacitance indicatormeans may be any device connected across the plates 126 and of theparallel plate capacitor which will respond to capacitance changes assmall as onetenth picofarad. Such a capacitance indicator means which iscomprised in a preferred embodiment of this invention is illustrated inFIGS. 6-9.

FIG. 6 shows a tuned collector, tuned base transistor oscillator(hereinbelow referred to as a T.C.T.B. oscillator) designated generallyat 302 for use in the capacitance indicator means which is comprised inan embodiment of the present invention. The T.C.T.B. oscillator 302 isof standard design and comprises a transistor 304 which in a preferredembodiment of the present invention is a 2N2925 transistor manufacturedby General Electric Co. The transistor 304 comprises a collector 306, anemitter 308 and a base 310. An inductance 312 connected in parallel witha capacitance 314 comprise the collector tuned circuit, designatedgenerally at 316, of the T.C.T.B. oscillator 302. An inductance 318connected in parallel with a capacitance 320 comprise the base tunedcircuit, designated generally at 322, of the T.C.T.B. oscillator 302. Aresis tor 324 connecting the collector tuned circuit 316 to the base 310of transistor 304 comprises the base bias resistor. A capacitance 326connecting the base tuned circuit 322 to the base 310 of transistor 304comprises the base coupling capacitor.

The collector inductance 312 and the base inductance 318 areelectromagnetically coupled by virtue of their proximity.

The base tuned circuit 322 of T.C.T.B. oscillator 302 has nearly thesame resonant frequency as the collector tuned circuit 316 of T.C.T.B.oscillator 302. As the base tuned circuit 322 is adjusted away from thismutually resonant point by adding capacitance to capacitance 320, thecurrent of collector 306 of transistor 304 changes in an essentiallyS-shaped manner as a function of capacitance added to capacitance 320.

FIG. 8 is a graph of the relative collector 306 current as a function ofcapacitance added to capacitance 320 in base tuned circuit 322. In thecentral portion of the S-shaped curve of FIG. 8 the relative collector306 current is approximately linear with respect to small variations ofcapacitance 320 in the base tuned circuit 322. In other words, in thislinear region of operation of the T.C.T.B. oscillator 302 with respectto relative collector 306 current as a function of capacitance added tocapacitance 320, any change in the capacitance 320 within the linearrange will result in a change of relative collector 306 current which isproportional to the change in capacitance 320. This relationship may beexpressed as follows:

AI =K AC where AI is a change in relative collector 306 current, AC,, isa change in capacitance 320 and K is a proportionality constant withrespect to capacitance. Thus the T.C.T.B. oscillator 302, within theabove described linear range of operation, acts as a current sourcewhich produces a current (collector 306 current) which increases inproportion to an unknown capacitance added to the base tuned circuit322.

It can be demonstrated experimentally that the departures from linearityare small if an accurately calibrated variable capacitor, General RadioCo. type 722-S54, for example, is connected in parallel with the basetuned circuit 322. This was done with an embodiment of the circuitherein described-Said circuit was adjusted so that a full scaledeflection of meter 338 corresponded to a capacitance change of 10.00picofarads. Known capacitance changes were then The average erroris'less than 1 percent, only part of which is ascribable tonon-linearity.

It is to be noted that collector 306 current is also proportional to anunknown inductance added to the base tuned circuit 322 when the T.C.T.B.oscillator 302 is within the linear range of operation with respect torelative collector 306 current as a function of inductance added toinductance 318 in base tuned circuit 322. This relationship may beexpressed as follows: AI =K; AL where AI is a change in relativecollector 306 current, AL is a change in inductance 318 and K, is aproportionality constant with respect to inductance.

Thus, the T.C.T.B. oscillator 302 can be utilized to measure an unknowninductance or capacitance which is added to the base tuned circuit 322by determining the appropriate proportionality constant K or K, and

by measuring the resultant change in collector 306 current.

FIG. 7 and FIG. 9 show the T.C.T.B. oscillator 302 connected in aWheatstone bridge circuit. The Wheatstone bridge circuit ofFlGS. 7 and 9is of generally standard design and comprises R.F. bypass capacitors 328and 330. The Wheatstone bridge circuit further comprises bridgeresistors 332, 334 and 336. A meter 338 measures the bridge unbalancevoltage and a potentiometer 340 is used to adjust the meter sensitivity.The meter 338 comprised in a preferred embodiment of the presentinvention is a product of Assembly Products Inc. Model 502 which has a100 microampere range. A potentiometer 342 is usedas a zero adjust forthe meter 338.

As described hereinabove,-the T.C.T.B. oscillator 302 acts as a currentsource, developing a voltage across bridge resistor 332. a r

In a preferred embodiment of the present invention used in practice, thecapacitance indicator means comprised in the present invention isadapted to indicate changes of from zero to ten picofarads on the meterscaleof meter 338. Altemately, a decading switch can be added to switchcapacitance out of the circuit in successive increments of 10 picofaradseach. This allows the capacitance indicator means to measure changes ofup to approximately picofarads, with the component values listed below,while stillretaining the ac-- curacy and resolution of the ten picofaraddevice.

In its application as a ten picofarad device the capacitance indicatormeans of this invention comprises a variable. capacitance 344 inparallel with capacitance 320 of the base tuned circuit 322. Thevariable capacitance 344 is set for a zero meter current reading onmeter 338 with no external capacitance added to the base tuned circuit322, and potentiometer 340 is adjusted for meter full-scale current onmeter 338 when'a l0'picofarad capacitance is added to the base tunedcircuit 322 in parallel with capacitance 320 and 344. In a preferredembodiment of the present invention measuring zero to 10 picofarads onthe meter scale of meter 338, the transistor 304 is arbitrarily chosento be a transistor manufactured by the General Electric Co. anddesignated by the General Electric Co. as 2N2925. As describedhereinabove, the transistor 304 is incorporated into the T.C.T.B.oscillator 302. The collector tuned circuit 316 and the base tunedcircuit 320 are selected so that the transistor 304 will operate in thecentral portion of the S-shaped curve of FIG. 8. Thus, the inductance312 is chosen to be approximately 8.2 microhenries, a capacitance 314 ischosen to be approximately 220 picofarads, an inductance 318 is chosento be approximately 8.2 microhenries and the capacitance 320 is chosento be approximately 40 picofarads. In addition, the base bias resistor324 is chosen to be approximately 560 K and the base coupling capacitor326 is chosen to be approximately 0.01 microfarads. Furthermore,. thevariable capacitance 344 which aids in tuning the base tuned circuit 322is chosen to vary over a range approximately 1-12 picofarads.

As described hereinabove, the collector 306 current of the transistor304 is measured in a Wheatstone bridge circuit. For the choice of valuesof the components of the T.C.T.B. oscillator 302 given above, the bridgeresistors 332, 334 and 336 are chosen to be 2.4 K. The potentiometer 340is chosen to have approximately a 5 K span and the potentiometer 342 ischosen to have approximately a 100 ohm span. The two R.F. bypasscapacitors 328 and 330 are chosen to have approximately 0.05 and 0.01mic'rofarads, respectively.

The foregoing description in conjunction with the drawing is consideredillustrative only of the principles of the invention. Further, sincenumerous modifications and changes will readily occur to those skilledin the art, it is not desired to limit the invention to the exactconstruction and operation shown and described, and accordingly allsuitable modifications and equivalents may be resorted to, fallingwithin the scope of the invention as claimed.

What is claimed is:

1. Apparatus for measuring the quantity of carbon in smoke filtershaving carbon-bearing and non-carbonbearing portions, said apparatuscomprising:

second electrodes comprise parallel conductive plates having asubstantially flat configuration.

additional electrode electrically connectable to said measuring meansand said second electrode and having conductive protrusions on onesurface thereof and being disposable in a spaced facing relationshipwith said second electrode, with said protrusions of said ad ditionalelectrode extending toward said second electrode, in the regions of saidfilter rods which are noncarbon-bearing.

a. first and second electrodes disposable in a spaced facing relation toeach other to accommodate placement of the filters in a patterntherebetween, said first electrode having conductive protrusions on onesurface thereof extending toward said second electrode adjacent thecarbon-bearing regions of the filters for establishing in the spacebetween said electrodes an electric field when an electric potentialdifference is applied between said electrodes, said electric field beingrelatively concentrated in the regions of the carbon-bearing portions ofthe filters with respect to the concentration of said field in saidnon-carbon-bearing portions of said filters and being directed primarilyperpendicular to said electrodes; and

b. means electrically connectable to said first and second electrodesfor measuring the electrical capacitance between said electrodes.

2. The apparatus of claim 1, in which: said first and 3. The apparatusof claim 2, further comprising:

means for maintaining filters having rod configuration in a pattern ofparallel adjacency with the carbon-bearing regions of the filtersrespectively aligned adjacent one another. i

4. The apparatus of claim 3, in which said protru- 5. The apparatus ofclaim 4, further comprising an 6. The apparatus of claim 5, in whichsaid protrusions on said additional electrode comprise conductiveparallel ridges.

7. The apparatus of claim 2, having means for holding the filters insaid pattern, said means comprising:

means for maintaining filters having rod configuration in a pattern ofparallel adjacency with the carbon-bearing portions of the filtersrespectively aligned adjacent one another.

9. The apparatus of claim 8, having means for holding the filter rods insaid pattern, said means comprising:

a. a pair of non-conductive side walls disposed adjacent opposite sidesof said first and second'electrodes; and

b. a non-conductive'end wall disposed adjacent one end of said first andsecond electrodes, whereby said side walls and said end wall cooperatein forming a frame to maintain the filter rods in said pattern.

10. The apparatus of claim 8 further comprising:

a vibrator to vibrate the filter rods when the rods are disposed in saidpattern, whereby the carbon in thecarbon-bearing portions of the filterrods is settled by means of the vibrations created by said vibrator. V

11. The apparatus of claim 1, further comprising: hinge means forhingedly mounting one of said first and second electrodes 'adjacenttheother of said first and second electrodes, whereby said one of saidfirst and second electrodes may be selectively rotated into and out ofsaid spaced facing relation with said other of said first and secondelectrodes.

12. The apparatus of claim 11, further comprising: fastening meansattached to at least one of said first and second electrodes for lockingsaid one of said first and second electrodes in said spaced facingrelationship with the said other of said first and second electrodes.

13. The apparatus of claim 1, wherein said protrusions compriseconductive parallel spaced ridges.

14. The apparatus of claim l3, further comprising an additionalelectrode electrically connectable to said measuring means and saidsecond electrode and-having conductive protrusions on one surfacethereof and being disposable in a spaced facing relationship with saidsecond electrode, with said protrusions of said additional electrodeextending toward said second electrode said protrusions being spaced tobe disposed adjacent said non-carbon-bearing portions at said filterswhen said second electrode and said additional electrode are in saidspaced facing relation.

15. The apparatus of claim 14, in which said protrusions on saidadditional electrode comprise spaced parallel ridges.

16. The apparatus of claim 1, further comprising: a vibrator attached tosaid apparatus to vibrate the filters when the filters are disposed insaid pattern, whereby the carbon in the carbon-bearing portions of thefilters is settled by means of the vibrations created by said vibrator.

17. The apparatus of claim 1, in which said capacitance measuring meanscomprises:

a. an electric circuit connected to said first and second electrodeshaving means for generating a reference electric current in saidelectric circuit, the magnitude of said reference current being asubstantially linear function of the magnitude of the capacitancebetween said first and second electrodes over a range of magnitude ofsaid capacitance; and

b. means to measure the magnitude of said reference current.

18. The apparatus of claim 17, in which said electric circuit comprises:

a. a transistor oscillator having a tuned base circuit and atunedcollector circuit connected thereto, said first and secondelectrodes being connected as a capacitor in said tuned base circuit,said reference current being the collector current of said transistor.

19. The apparatus of claim 18, wherein:

a. said tuned collector circuit comprises a first inductive element anda capacitive element connected therein in parallel; and

b. said tuned base circuit comprises a second inductive elementconnected in said base circuit in parallel with said first and secondelectrodes.

20. The apparatus of claim 19, in which said means for measuring saidreference current comprises:

a Wheatstone bridge circuit into an arm of which the portion of saidtuned collector circuit comprising said first inductive element and saidcapacitive element is connected, to measure and indicate variations insaid reference current across said first inductive element and saidcapacitive element.

21. An apparatus to measure the relative quantity of carbon present insmoke filter rods, the filter rods including carbon-bearing regions andnon-carbon-bearing regions, each carbon-bearing region thereof includinga chamber partially filled with carbon granules, said apparatuscomprising:

a. a parallel plate capacitor comprising a first conductive plate and asecond conductive plate, said first conductive plate having spacedconductive ridges disposed on one side thereof, said first conductiveplate being disposable in a position in which said spaced conductiveridges face said second conductive plate;

b. means adapted to receive rods of filter material between said firstand second conductive plates, and to maintain the rods in a parallelrelationship, said means comprising a first side wall attached to saidsecond conductive plate, a second side wall attached to said secondconductive plate, parallel to said first side wall at a distance fromsaid first side wall greater than the length of the rods of filtermaterial to be tested, and at least one end wall disposed between saidfirst and second side walls forming with said first and second sidewalls and said second conductive plate a frame to maintain rods offilter material to be tested parallel to and between said first andsecond conductive plates; 7

c. a vibrator adaptedvto vibrate said rods of filter material toeffectuate a relatively consistent array of the carbon granules withinthe chambers from one measurement to another;

d. a third conductive plate having spaced ridges disposed on one sidethereof, said third conductive plate being disposable in a positionhaving said spaced ridges facing said second conductive plate, saidspaced ridges comprising electrically conductive material and beingspaced to dispose said ridges adjacent said non-carbon-bearing regionsof the rods of filter material to be tested;

a capacitance measuring and indicating device electrically connected tosaid parallel plate capacitor and said third conductive plate havingmeans adapted to measure change in capacitance of said parallel platecapacitor when the rods of filter material to be tested are disposedtherein and to indicate the carbon content of the rods; and

f. a power source electrically connected to one of said first and secondconductive plates adapted to apply electric potential difference betweensaid first and second conductive plates.

22. An apparatus for measuring the relative quantity of carbon presentin smoke filter rods, the filter rods having carbon-bearing regions eachincluding a chamber partially filled with carbon granules, andnoncarbon-bearing regions, said apparatus comprising a. aframe; y

b. a first electrically conductive capacitor plate rigidly fixed to saidframe;

a second electrically conductive capacitor plate rotatably mounted tosaid frame and adapted to be rotated into a spaced substantiallyparallel facing relation with said first conductive plate, said secondconductive plate comprising spaced ridges disposed on its side adjacentsaid first conductive plate, said spaced ridges comprising electricallyconductive material and being spaced to dispose said ridges adjacentsaid carbon-bearing regions of the filter rods when said secondconductive plate is disposed in its spaced substantially parallelrelation with said first conductive plate;

. means disposed between said first and second conductive plates toreceive the filter rods to be tested and to maintain the rods parallelto and between said first and second conductive plates when said firstconductive plate 'is disposed in its spaced parallel facing relationwith said second conductive plate;

. spacer means disposed between said first and second conductive platesto maintain said first conductive plate parallel to said secondconductive plate;

f. a-vibrator attached to said frame to vibrate the filter rods toeffectuate a relatively consistent array of carbon granules within thechambers of the filter rods from one measurement to another;

g. a third electrically conductive plate rotatably mounted to said frameto be rotatable into a spaced parallel facing relationship with saidfirst conductive plate, said third conductive plate having spaced ridgesdisposed on its side adjacent said second conductive plate, said spacedridges comprising electrically conductive material and being spaced todispose said ridges adjacent said noncarbon-bearing regions of the rodsof filter material when said third conductive plate is in its saidspaced parallel facing relationship with said first conductive plate;

h. a capacitance measuring and indicating means electrically connectableto said first, second and third conductive plates having'means adaptedto measure the change in capacitance between one of said first andsecond conductive plates and said first and third conductive plates,when the rods of filter material to be tested are disposed therebetweenand to indicate the carbon content of the rods; and

i. a power source electrically connected to the said capacitancemeasuring and indicating means-to electrically activate said measuringand indicating means. v I

23. A method of measuring the quantity of carbon present in smokefilters having carbon-bearing regions, and non-carbon-bearing regions,said method comprising the steps of:

* a. placing the filters between the electrodes of a capacitor;

b. applying electric potential difference between said electrodes toselectively generate an electric field in the said carbon-bearingregions substantially stronger than the electric field in the saidnon-carhon-bearing regions;

c. measuring the capacitance of said capacitor with the filters insertedtherebetween; and

d. comparing the measured capacitance with the capacitance of thecapacitor when smoke filters having a predetermined quantity of carbonare present in said capacitor.

24. The method of claim 23 wherein the step of measuring capacitancecomprises the steps of:

a. connecting said capacitor in parallel with an inductive element intothe tuned base circuit of a tuned collector, tuned base transistoroscillator which produces a reference collector current on actuationwhich is substantially proportional to the value of the capacitance ofsaid capacitor;

b. actuating said oscillator to produce said reference current;

c. measuring said reference current; and

d. comparing said reference current to that generated when filters withknown carbon content are placed between the electrodes of saidcapacitor.

25. The method of claim 23, comprising the further subsequent steps of:

a. applying an electric potential to said capacitor to selectivelygenerate an electric field in said noncarbon-bearing regions which issubstantially stronger than the electric field in said carbon-bearingregions, and then b. measuring the capacitance of said capacitor.

26. An apparatus for measuring the relative amount of carbon in smokefilters comprising:

a. a parallel plate capacitor having said plates positioned in asubstantially facing relation and spaced to accomodate placement of thefilters therebetween; and

b. means for measuring the capacitance of said parallel plate capacitorby generating a reference current which varies in proportion to thevalue of said capacitance, over a range of values of said capacitance,said measuring means including i. a transistor oscillator having a tunedcollector circuit having a first inductive element and a capacitiveelement connected therein in parallel, and a tuned base circuit having asecond inductive element connected therein in parallel with saidparallel plate capacitor; said collector and base circuits beinginductively coupled between said first and second inductive elements,said reference current being the current in said tuned collectorcircuit,

means for actuating said transistor oscillator, and

iii. means for measuring said reference current.

27. The apparatus of claim 26, in which said means for measuring saidreference current comprises:

a Wheatstone bridge circuit into an arm of which the portion of saidtuned collector circuit comprising said first inductive element and saidcapacitive element-is connected, to measure and indicate variations insaid reference current across said first inductive element and saidcapacitive element.

UNITED STATES PATENT OFFICE CERTIFICATE OF COECTKON Patent No. 3,696,299 Dated October 3, 1972 (Ser. No. 8,603) Filed (February 4, 1970)Inventor($) James O. Pullman and William R. Weidlich RELATIVE CARBONCONTENT ME F R M It is certified that error appears in theaboveidentified patent I and that said Letters Patent are herebycorrected as shown below:

Correct number of reference Patent No. 2,739,284.

Signed and sealed this 10th day of April 1973.

(SEAL) Attest:

EDWARD M.PLETCHER,JR.- ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents FORM PO1050(10-69) USCOMM-DC 60376-P6s 1* u.s. GOVERNMENTPRINTING OFFICE: 1969 o-3ss-3s4

1. Apparatus for measuring the quantity of carbon in smoke filtershaving carbon-bearing and non-carbon-bearing portions, said apparatuscomprising: a. first and second electrodes disposable in a spaced facingrelation to each other to accommodate placement of the filters in apattern therebetween, said first electrode having conductive protrusionson one surface thereof extending toward said second electrode adjacentthe carbon-bearing regions of the filters for establishing in the spacebetween said electrodes an electric field when an electric potentialdifference is applied between said electrodes, said electric field beingrelatively concentrated in the regions of the carbon-bearing portions ofthe filters with respect to the concentration of said field in saidnon-carbon-bearing portions of said filters and being directed primarilyperpendicular to said electrodes; and b. means electrically connectableto said first and second electrodes for measuring the electricalcapacitance between said electrodes.
 2. The apparatus of claim 1, inwhich: said first and second electrodes comprise parallel conductiveplates having a substantially flat configuration.
 3. The apparatus ofclaim 2, further comprising: means for maintaining filters having rodconfiguration in a pattern of parallel adjacency with the carbon-bearingregions of the filters respectively aligned adjacent one another.
 4. Theapparatus of claim 3, in which said protrusions comprise conductiveparallel ridges, said ridges positioned to be disposed adjacent saidaligned carbon-bearing portions of the filter rods when said first andsecond electrodes are disposed in said spaced facing relationship. 5.The apparatus of claim 4, further comprising an additional electrodeelectrically connectable to said measuring means and sAid secondelectrode and having conductive protrusions on one surface thereof andbeing disposable in a spaced facing relationship with said secondelectrode, with said protrusions of said additional electrode extendingtoward said second electrode, in the regions of said filter rods whichare non-carbon-bearing.
 6. The apparatus of claim 5, in which saidprotrusions on said additional electrode comprise conductive parallelridges.
 7. The apparatus of claim 2, having means for holding thefilters in said pattern, said means comprising: a. a pair ofnon-conductive side walls disposed adjacent opposite sides of said firstand second electrodes; and b. a non-conductive end wall disposedadjacent one end of said electrodes, whereby said side walls and saidend wall cooperate in forming a frame to maintain the filters disposedin said pattern.
 8. The apparatus of claim 1, further comprising: meansfor maintaining filters having rod configuration in a pattern ofparallel adjacency with the carbon-bearing portions of the filtersrespectively aligned adjacent one another.
 9. The apparatus of claim 8,having means for holding the filter rods in said pattern, said meanscomprising: a. a pair of non-conductive side walls disposed adjacentopposite sides of said first and second electrodes; and b. anon-conductive end wall disposed adjacent one end of said first andsecond electrodes, whereby said side walls and said end wall cooperatein forming a frame to maintain the filter rods in said pattern.
 10. Theapparatus of claim 8, further comprising: a vibrator to vibrate thefilter rods when the rods are disposed in said pattern, whereby thecarbon in the carbon-bearing portions of the filter rods is settled bymeans of the vibrations created by said vibrator.
 11. The apparatus ofclaim 1, further comprising: hinge means for hingedly mounting one ofsaid first and second electrodes adjacent the other of said first andsecond electrodes, whereby said one of said first and second electrodesmay be selectively rotated into and out of said spaced facing relationwith said other of said first and second electrodes.
 12. The apparatusof claim 11, further comprising: fastening means attached to at leastone of said first and second electrodes for locking said one of saidfirst and second electrodes in said spaced facing relationship with thesaid other of said first and second electrodes.
 13. The apparatus ofclaim 1, wherein said protrusions comprise conductive parallel spacedridges.
 14. The apparatus of claim 13, further comprising an additionalelectrode electrically connectable to said measuring means and saidsecond electrode and having conductive protrusions on one surfacethereof and being disposable in a spaced facing relationship with saidsecond electrode, with said protrusions of said additional electrodeextending toward said second electrode said protrusions being spaced tobe disposed adjacent said non-carbon-bearing portions at said filterswhen said second electrode and said additional electrode are in saidspaced facing relation.
 15. The apparatus of claim 14, in which saidprotrusions on said additional electrode comprise spaced parallelridges.
 16. The apparatus of claim 1, further comprising: a vibratorattached to said apparatus to vibrate the filters when the filters aredisposed in said pattern, whereby the carbon in the carbon-bearingportions of the filters is settled by means of the vibrations created bysaid vibrator.
 17. The apparatus of claim 1, in which said capacitancemeasuring means comprises: a. an electric circuit connected to saidfirst and second electrodes having means for generating a referenceelectric current in said electric circuit, the magnitude of saidreference current being a substantially linear function of the magnitudeof the capacitance between said first and second electrodes over a rangeof magnitude of said capacitance; and b. means to measure the magnitudeof said rEference current.
 18. The apparatus of claim 17, in which saidelectric circuit comprises: a. a transistor oscillator having a tunedbase circuit and a tuned collector circuit connected thereto, said firstand second electrodes being connected as a capacitor in said tuned basecircuit, said reference current being the collector current of saidtransistor.
 19. The apparatus of claim 18, wherein: a. said tunedcollector circuit comprises a first inductive element and a capacitiveelement connected therein in parallel; and b. said tuned base circuitcomprises a second inductive element connected in said base circuit inparallel with said first and second electrodes.
 20. The apparatus ofclaim 19, in which said means for measuring said reference currentcomprises: a Wheatstone bridge circuit into an arm of which the portionof said tuned collector circuit comprising said first inductive elementand said capacitive element is connected, to measure and indicatevariations in said reference current across said first inductive elementand said capacitive element.
 21. An apparatus to measure the relativequantity of carbon present in smoke filter rods, the filter rodsincluding carbon-bearing regions and non-carbon-bearing regions, eachcarbon-bearing region thereof including a chamber partially filled withcarbon granules, said apparatus comprising: a. a parallel platecapacitor comprising a first conductive plate and a second conductiveplate, said first conductive plate having spaced conductive ridgesdisposed on one side thereof, said first conductive plate beingdisposable in a position in which said spaced conductive ridges facesaid second conductive plate; b. means adapted to receive rods of filtermaterial between said first and second conductive plates, and tomaintain the rods in a parallel relationship, said means comprising afirst side wall attached to said second conductive plate, a second sidewall attached to said second conductive plate, parallel to said firstside wall at a distance from said first side wall greater than thelength of the rods of filter material to be tested, and at least one endwall disposed between said first and second side walls forming with saidfirst and second side walls and said second conductive plate a frame tomaintain rods of filter material to be tested parallel to and betweensaid first and second conductive plates; c. a vibrator adapted tovibrate said rods of filter material to effectuate a relativelyconsistent array of the carbon granules within the chambers from onemeasurement to another; d. a third conductive plate having spaced ridgesdisposed on one side thereof, said third conductive plate beingdisposable in a position having said spaced ridges facing said secondconductive plate, said spaced ridges comprising electrically conductivematerial and being spaced to dispose said ridges adjacent saidnon-carbon-bearing regions of the rods of filter material to be tested;e. a capacitance measuring and indicating device electrically connectedto said parallel plate capacitor and said third conductive plate havingmeans adapted to measure change in capacitance of said parallel platecapacitor when the rods of filter material to be tested are disposedtherein and to indicate the carbon content of the rods; and f. a powersource electrically connected to one of said first and second conductiveplates adapted to apply electric potential difference between said firstand second conductive plates.
 22. An apparatus for measuring therelative quantity of carbon present in smoke filter rods, the filterrods having carbon-bearing regions each including a chamber partiallyfilled with carbon granules, and non-carbon-bearing regions, saidapparatus comprising a. a frame; b. a first electrically conductivecapacitor plate rigidly fixed to said frame; c. a second electricallyconductive capacitor plate rotatably mounted to said frame and adaptedto be rotated inTo a spaced substantially parallel facing relation withsaid first conductive plate, said second conductive plate comprisingspaced ridges disposed on its side adjacent said first conductive plate,said spaced ridges comprising electrically conductive material and beingspaced to dispose said ridges adjacent said carbon-bearing regions ofthe filter rods when said second conductive plate is disposed in itsspaced substantially parallel relation with said first conductive plate;d. means disposed between said first and second conductive plates toreceive the filter rods to be tested and to maintain the rods parallelto and between said first and second conductive plates when said firstconductive plate is disposed in its spaced parallel facing relation withsaid second conductive plate; e. spacer means disposed between saidfirst and second conductive plates to maintain said first conductiveplate parallel to said second conductive plate; f. a vibrator attachedto said frame to vibrate the filter rods to effectuate a relativelyconsistent array of carbon granules within the chambers of the filterrods from one measurement to another; g. a third electrically conductiveplate rotatably mounted to said frame to be rotatable into a spacedparallel facing relationship with said first conductive plate, saidthird conductive plate having spaced ridges disposed on its sideadjacent said second conductive plate, said spaced ridges comprisingelectrically conductive material and being spaced to dispose said ridgesadjacent said non-carbon-bearing regions of the rods of filter materialwhen said third conductive plate is in its said spaced parallel facingrelationship with said first conductive plate; h. a capacitancemeasuring and indicating means electrically connectable to said first,second and third conductive plates having means adapted to measure thechange in capacitance between one of said first and second conductiveplates and said first and third conductive plates, when the rods offilter material to be tested are disposed therebetween and to indicatethe carbon content of the rods; and i. a power source electricallyconnected to the said capacitance measuring and indicating means toelectrically activate said measuring and indicating means.
 23. A methodof measuring the quantity of carbon present in smoke filters havingcarbon-bearing regions, and non-carbon-bearing regions, said methodcomprising the steps of: a. placing the filters between the electrodesof a capacitor; b. applying electric potential difference between saidelectrodes to selectively generate an electric field in the saidcarbon-bearing regions substantially stronger than the electric field inthe said non-carbon-bearing regions; c. measuring the capacitance ofsaid capacitor with the filters inserted therebetween; and d. comparingthe measured capacitance with the capacitance of the capacitor whensmoke filters having a predetermined quantity of carbon are present insaid capacitor.
 24. The method of claim 23 wherein the step of measuringcapacitance comprises the steps of: a. connecting said capacitor inparallel with an inductive element into the tuned base circuit of atuned collector, tuned base transistor oscillator which produces areference collector current on actuation which is substantiallyproportional to the value of the capacitance of said capacitor; b.actuating said oscillator to produce said reference current; c.measuring said reference current; and d. comparing said referencecurrent to that generated when filters with known carbon content areplaced between the electrodes of said capacitor.
 25. The method of claim23, comprising the further subsequent steps of: a. applying an electricpotential to said capacitor to selectively generate an electric field insaid non-carbon-bearing regions which is substantially stronger than theelectric field in said carbon-bearing regions, and then b. measuring theCapacitance of said capacitor.
 26. An apparatus for measuring therelative amount of carbon in smoke filters comprising: a. a parallelplate capacitor having said plates positioned in a substantially facingrelation and spaced to accomodate placement of the filters therebetween;and b. means for measuring the capacitance of said parallel platecapacitor by generating a reference current which varies in proportionto the value of said capacitance, over a range of values of saidcapacitance, said measuring means including i. a transistor oscillatorhaving a tuned collector circuit having a first inductive element and acapacitive element connected therein in parallel, and a tuned basecircuit having a second inductive element connected therein in parallelwith said parallel plate capacitor; said collector and base circuitsbeing inductively coupled between said first and second inductiveelements, said reference current being the current in said tunedcollector circuit, ii. means for actuating said transistor oscillator,and iii. means for measuring said reference current.
 27. The apparatusof claim 26, in which said means for measuring said reference currentcomprises: a Wheatstone bridge circuit into an arm of which the portionof said tuned collector circuit comprising said first inductive elementand said capacitive element is connected, to measure and indicatevariations in said reference current across said first inductive elementand said capacitive element.